Composite interposer for BGA packages

ABSTRACT

An interposer in a BGA or similar package includes a polymide core and a filler of thermally conductive, electrically nonconductive filler. The interposer has a higher thermal conductivity as compared to conventional interposers, thereby increasing the thermal dissipation through the interposer and enabling the device to cool more efficiently. The filler also reduces the coefficient of thermal expansion of the interposer to more closely match the die and reduce stresses. Furthermore, the filler increases the rigidity of the interposer, thereby enabling the interposer to be handled and carried more easily, for example, without a metal frame carrier.

This application is a divisional application of U.S. patent applicationSer. No. 09/652,977, filed Aug. 31, 2000 now U.S. Pat. No. 6,710,456,the entire contents of which are hereby expressly incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to integrated circuit packages, and moreparticularly, to an interposer for a ball grid array (BGA) packagehaving high thermal dissipation, a low coefficient of thermal expansion(CTE) and a high Young's modulus.

2. Description of the Related Art

In the last few decades, the electronics industry has literallytransformed the world. Electronic products are used by, or affect thedaily lives of, a large segment of the world's population. For example,telephones, televisions, radios, personal computers (PCs), laptop PCs,palmtop PCs, PCs with built-in portable phones, cellular phones,wireless phones, pagers, modems and video camcorders, are just a few ofthe electronic products that have been developed in recent years andwhich have been made smaller and more compact, while providing morefunctions than ever before and/or enhanced functions. The integratedcircuit (IC) chip and the more efficient packaging of the IC chip haveplayed key roles in the success of these products.

The IC chip is not an isolated island. It must communicate with otherchips in a circuit through an Input/Output (I/O) system ofinterconnects. Moreover, the IC chip and its embedded circuitry aredelicate, and must therefore be protected in a package that can bothcarry and protect it. As a result, the major functions of the IC packageare: (1) to provide a path for the electrical current that powers thecircuits on the chip; (2) to distribute the signals on to and off of thechip; (3) to remove the heat generated by the circuit; and (4) tosupport and protect the chip from hostile environments.

As ICs become more complex and printed circuit boards become morecrowded, IC packages continually need more leads or pins while theirfootprints consume smaller and smaller areas. In an effort to meet thesedemands, developers created the ball grid array (BGA) package.

A typical BGA package includes an IC affixed to a flexible polyimidetape or interposer. A very thin conductor or wire bond connects a pad onthe IC to a conductive trace on the polyimide tape. The conductive traceis routed to a solder ball. The solder ball is one of an array of solderballs mounted to the opposite side of the polyimide tape and protrudingfrom the bottom of the BGA package. These solder balls interconnect withan array of pads located on a substrate, such as a printed circuitboard. Accordingly, the typical BGA package electrically connects eachpad on an IC to a pad on a printed circuit board.

A variation of the BGA package that has been introduced recently is theArea Tape Automated Bonding (ATAB) Ball Grid Array (BGA) package, ormore commonly referred to as simply the Tape Ball Grid Array (TBGA)package. The TBGA package advantageously provides high lead counts, isthin, is lightweight, has high electrical and thermal performance, andhas a BGA surface mount. The conventional TBGA package includes a tapecontaining a polyimide dielectric. At least one layer of the tape isformed into traces or conductors that interconnect a chip to a printedcircuit board (PCB). See John H. Lau (Ed.), Ball Grid Array Technology,Chapter 14, “Area Tape Automated Bonding Ball Grid Array Technology”(McGraw-Hill, 1995), incorporated herein by reference.

One particular type of BGA package developed by Tessera is themicro-ball grid array (μBGA) package. The basic package typicallyincludes a package interposer that is a 25 μm thick polyimide film withdouble-sided copper (Cu). One side of the Cu serves as a ground plane,which the other side has signal traces for I/O redistribution. A layerof silicone elastomer is positioned between the chip and the substrate.This compliant layer typically has a thickness of 150 μm. Thefirst-level interconnects of the μBGA are flexible ribbons which aretypically bonded on aluminum (Al) bond pads on the chip by a single-shotthermosonic process. The ribbons are typically 25 μm-wide soft gold (Au)leads with a thickness of 20 to 25 μm, bonded in a lazy-S shape so thatthey may accommodate any deformation due to thermal expansion. In orderto protect the bonded leads, an encapsulant such as a silicone materialis dispensed from the back side (between the chip and the interposer)after the lead bonding is completed. The package terminals of the μBGAmay be plated bumps, solder balls or solid-core metal spheres. Furtherdetails describing the typical μBGA package may be found in John H. Lau,Chip Scale Package, Chapter 16, “Tessera's Micro-Ball Grid Array (μBGA)”(McGraw-Hill, 1999), incorporated herein by reference.

In a typical μBGA manufacturing process, the flexible tape interposer isfirst provided and tailored from a reel to mount as strips onto a metalframe. The elastomer layer is applied to the tape, and an adhesivematerial is deposited for die attachment. Die attachment is performedwith an automated pick-and-place machine. Subsequently, ribbon leads arebonded to the Al die pads by a thermosonic process. Once the leadbonding is finished, a dry film resist is laminated to the interposerusing a vacuum system. The encapsulant is dispensed from the back side,and the curing is performed to complete the encapsulation. Thesubsequent procedures include dry film exposure and developing,solder-ball attachment and reflow, cleaning, marking, and packagesingulation. Further details are described in Chapter 16 of John Lau'sChip Scale Package referenced above.

One problem with integrated circuits, including BGA packages, is thatthey require precise temperature control for efficient operation. Thus,if a package runs too hot, the heat can affect the performance andtiming of the device. Accordingly, there is a need for an effective wayto maintain control over the temperature of a device and keep it cool.

Another problem in BGA and similar packages is the mismatch incoefficient of thermal expansion (CTE) between the die and the tape orinterposer containing the polyimide dielectric. The polyimide tapetypically has a much higher coefficient than that of the die to whichthe tape is bonded. For instance, a die having a CTE of about 3 ppm/° C.may be coupled to a polyimide tape interposer having a CTE of about 20ppm/° C. or more. This mismatch causes the tape to expand and shrinkmore rapidly than the die, thereby creating stress on the conductiveleads connecting the solder ball array to the die. This stress can leadto breakage of the wire and a corresponding loss of electricalconnection between the IC pads. The mismatch in CTE between theinterposer and the die can also lead to delamination of the die attachor elastomer layer found therebetween. These problems result in loweryield rates and increase the overall cost of package manufacture.

SUMMARY OF THE INVENTION

Briefly stated, the preferred embodiments of the present inventionaddress these and other problems by providing an interposer in a BGA orsimilar package comprising a polymide core and a filler of thermallyconductive, electrically nonconductive filler. This improved interposerhas a higher thermal conductivity as compared to conventionalinterposers, thereby increasing the thermal dissipation through theinterposer and enabling the device to cool more efficiently. The filleralso reduces the CTE of the interposer to more closely match the die andreduce stresses. Furthermore, the filler increases the rigidity of theinterposer, thereby enabling the interposer to be handled and carriedmore easily, for example, without a metal frame carrier.

In one aspect of the present invention, an interposer for an integratedcircuit package is provided. The interposer comprises a polyimide coreand a thermally conductive, electrically nonconductive filler.Preferably, the interposer includes between about 10% and 95% filler byweight. More preferably, this filler may be either boron nitride oralumina. The filler advantageously increases the thermal conductivity ofthe interposer by at least about 5% as compared to that of the polyimidecore alone. The filler also advantageously reduces the coefficient ofthermal expansion of the interposer by at least about 10% as compared tothe polyimide core alone. Furthermore, the filler advantageouslyincreases the modulus of the interposer by at least about 10% ascompared to the modulus of the polyimide core alone.

In another aspect of the present invention, a flexible tape forconnecting a die to a plurality of package terminals is provided. Thetape comprises a polyimide core and a filler material. The fillermaterial increases the thermal conductivity of the tape as compared tothe thermal conductivity of the polyimide core alone.

In another aspect of the present invention, an integrated circuitpackage is provided. This package comprises a die, a die attach layerover the die, an array of package terminals over the die attach layer,and a composite tape between the die attach layer and the array ofpackage terminals. The composite tape comprises a polyimide core and athermally conductive filler embedded therein. The composite tape furthercomprises a layer of metal wire electrically connected to the die.

In another aspect of the present invention, a method of increasing thethermal conductivity of a flexible tape for use in an integrated circuitis provided. This method comprises adding a filler of thermallyconductive material to a polyimide film. In one preferred embodiment,the filler is added by blending the filler into a polyimide resin.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a bottom view of a face-down, fan-in package employing anexpansion lead, according to one embodiment of the present invention.

FIG. 1B shows a fragmentary cross-sectional view of a face-down, fan-inpackage employing an expansion lead, according to one embodiment of thepresent invention.

FIG. 1C shows a fragmentary cross-sectional view of a face-down, fan-inpackage employing an expansion lead having the leads on the secondsurface of the substrate, according to one embodiment of the presentinvention.

FIG. 1D shows a fragmentary cross-sectional view of a face-down, fan-inpackage employing an expansion lead wherein a compliant layer isdisposed between the face surface of the chip and the first surface ofthe substrate, according to one embodiment of the present invention.

FIG. 2 is a perspective view of a μBGA package.

FIG. 3 is a cross-sectional view of a μBGA package.

FIG. 4 is a cross-sectional view of a preferred interposer of thepresent invention.

FIG. 5 is a cross-sectional view of a first level package being attachedto a second level package.

FIG. 6 is a cross-sectional view of the first level package of FIG. 5,shown without the first level package case.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The embodiments described herein particularly relate to an interposerconnecting a die to a solder ball array in a μBGA package. However, itwill be appreciated that the principles of the present invention pertainnot only to μBGA technology, but also to other BGA, TBGA or flexiblecircuitry and other integrated circuit packaging. For example, theprinciples of the present invention are also applicable to Micron's BOC(Board-on-Chip) package.

As described in greater detail below, the preferred interposer comprisesa polyimide core and a thermally conductive, electrically nonconductivefiller. By adding filler to the polyimide core, the thermal conductivityof the interposer increases allowing the interposer to dissipate heatmore effectively. In addition, the coefficient of thermal expansion(CTE) of the interposer is reduced to more closely match the die.Moreover, the filler increases the rigidity of the interposer, makingthe interposer more durable to handling, which can eliminate the needfor the metal frame carrier process that is used with conventionalinterposers to attach the tape to the die.

FIGS. 1A and 1B show a face view and a fragmentary cross-sectional view,respectively, of a chip 10 having a plurality of chip contacts 20 on acontact bearing surface. A sheet-like dielectric chip carrier substrate30 overlies and is typically centrally located on the contact bearingsurface of the chip 10 so that the chip contacts 20 are exposed. Thesubstrate 30 may merely overlie the contact bearing surface of the chip10; however, typically, the substrate is adhesively attached to the chipsurface using a thin layer of adhesive material 80, as shown in FIG. 1B.

The substrate 30 may comprise a rigid or flexible material. Preferably,the substrate comprises a sheet of polyimide having a thicknessapproximately between 25 and 100 microns. The first surface of thesubstrate 30 has a plurality of conductive terminals 40 thereon. Theterminals 40 are electrically connected to a chip contact 20 throughrespective conductive leads 50 extending along the opposite side of thesubstrate and connected to the leads 50 through conductive vias 70.Alternately, the substrate may simply be removed so that solder ballterminals are placed directly onto the ends of the leads 50 withoutrequiring the conductive vias 70.

Each lead 50 has an expansion section 55 extending from an edge of thesubstrate 30. Each expansion section is bonded to a respective chipcontact 20, typically using conventional ultrasonic or thermosonicbonding apparatus. Each expansion section 55 is laterally curvedsubstantially parallel to the plane of the substrate 30 prior to thebonding operation. Preferably, each expansion section 55 laterallycurves at least twice in opposite directions (substantially “s” shaped)and may be curved more than twice. The leads 50 may further bedetachably connected to a supporting structure prior to bonding asdisclosed in U.S. Pat. Nos. 5,489,749 and 5,536,909, which are herebyincorporated by reference in their entirety.

Typically, the expansion sections 55 of the leads are encapsulated by asuitable encapsulant, such as silicone or epoxy, to protect them fromcontamination and damage. During operation of the packaged chip, theterminals are attached to a printed circuit board and the laterallycurved shape of the expansion sections 55 of the leads 50 helps tocompensate for the expansion and contraction of the chip during thermalcycling by having the ability to independently flex and bend. Theaforementioned encapsulant 60 supports the expansion sections 55 of theleads 50 as they flex and bend and further helps to spread the forcesacting on the leads. Further, a solder mask or coverlay may be placedover the exposed surface of the substrate 30 after the bonding andencapsulation steps such that only the terminals are exposed.

FIG. 1C shows a fragmentary cross-sectional view of an alternateembodiment in which the leads 50′ are located on the same side as theterminals 40; thus, not requiring the conductive vias 70 (shown in FIG.1B). A solder mask/coverlay is also used in the embodiment shown in FIG.1C because the leads 50 and the terminals 40 are on the same side of thesubstrate 30. The solder mask/coverlay provides a dielectric coatingensuring that the solder connecting the terminals to contacts on theprinted circuit board does not wick down the leads or short to othersoldered terminals.

FIG. 1D shows a fragmentary cross-sectional view of an alternateembodiment in which the thin layer of adhesive from FIG. 1B has beenreplaced with a thicker layer of compliant material 80′ to give addedcompensation for thermal mismatch, as disclosed in U.S. Pat. Nos.5,148,265 and 5,148,266, which are hereby incorporated by reference intheir entirety. The compliant material 80′ is typically about 50 to 200microns thick and comprises either a thermoset or a thermoplasticmaterial. The structure shown in FIG. 1D also allows the expansionsections 55 of the leads 50 to be shaped by the bonding operation sothat they are curved in a direction perpendicular to the lateral curveof the leads 50. As stated above, these laterally and vertically curvedleads are typically supported by the encapsulant 60 so as to spread theforces acting upon them during thermal cycling of the operationalpackage. Further details regarding these and other embodiments aredisclosed in U.S. Pat. No. 5,821,608, the entirety of which is herebyincorporated by reference.

FIGS. 2 and 3 illustrate one embodiment of the present invention inwhich a first level package 8 is provided, wherein like components arenumbered in accordance with FIGS. 1A-1D above. In the IC packagingindustry, it is common to refer to the placement of the IC chip within asuitable package as “1st level” packaging. The placement or mounting ofthe IC package on a suitable printed circuit board (PCB) or othersubstrate, is referred to as “2nd level” packaging. The interconnectionof the various PCBs or other carriers within an electronic system, e.g.,through use of a motherboard, is referred to as “3rd level” packaging.

The package 8 is preferably a ball grid array (BGA) package havingaplurality of solder balls 40 that interconnect the package to a printedcircuit board (see FIGS. 5 and 6). More preferably, the BGA package 8 isa TBGA package, and is even more preferably a μBGA package. As shown inFIGS. 2 and 3, in this package 8, a die or chip 10 is prepared forbonding with a second level package. As shown in FIG. 5, the integratedcircuit die 10 of the BGA package is mounted to a printed circuit board82 through solder pads 88 and enclosed by a rigid housing or lid 84,typically constructed from a molded plastic material. FIG. 6 illustratesan alternative embodiment of the μBGA package without a package case 84.

The die 10 will be understood by one of ordinary skill in the art to beone of many different types of integrated circuit. For example, the die10 can be from a wide range of integrated circuit products, such asmicroprocessors, co-processors, digital signal processors, graphicsprocessors, microcontrollers, memory devices, reprogrammable devices,programmable logic devices, and logic arrays, etc.

A die attach material 80 is provided over the central portion of the die10. A solder ball array 40 is provided over the die attach material. Thesolder ball array 40 serves to make the connection to the next-levelpackage. The die attach material 80 may be a silicone clastomer, or morepreferably, is an epoxy-modified elastomeric material such as describedin assignee's copending application entitled DIE ATTACH MATERIAL FORTBGA OR FLEXIBLE CIRCUITRY, U.S. patent application Ser. No. 09/471,071filed Dec. 21, 1999, the entirety of which is hereby incorporated byreference. The solder balls 40 are preferably relatively flexible andcan thus compensate for any lack of flatness in the printed circuitboard or package. Additionally, the solder balls are assembled in anarray, and thus provide a relatively high throughput. In one preferredembodiment, the solder balls are made of a tin/lead (SnPb) eutecticmaterial such as Sn63Pb37 and have a diameter of about 0.3 to 0.5 mm.

The tape or interposer 30 extends over the die attach material 80 toform a connection with the solder ball array 40. The bump pitch of thesolder balls 40 on the tape 30 can be as small as about 0.25 to 1 mm,and is more preferably about 0.5 mm. Leads 50 extend from the tape 30 toform a connection with the die 10 at die pads 20. The leads arepreferably made of Au wire, and are preferably bonded thermosonically ina lazy-S shape in expansion section 55 to accommodate deformation due tothermal expansion.

FIG. 4 illustrates more particularly in cross-section the interposer ortape 30. The tape 30 preferably includes a composite polyimide core 100and conductive traces 102 and 104, which are preferably made of copper.The polyimide core 100 preferably has a thickness of about 25 μm. Thecopper traces preferably have a thickness of about 12 μm. The corematerial is preferably Upilex-S, available from Ube, Japan.

The polyimide core 100 more preferably includes a thermally conductive,electronically nonconductive filler such as boron nitride or alumina. Itwill be appreciated that other inorganic materials having high thermalconductivity may also be used, including but not limited to silica,silicon nitride and aluminum nitride. In addition, metallic filler mayalso be used if it does not affect the circuitry signal integrity. Thefiller is preferably added by blending the filler material into thepolyimide resin system prior to film formation, such as in an extrusionmethod or a coating method. The filler preferably has an averagediameter of about 75 μm or less, more preferably less than about 500 μm.Filler may preferably be added to the core in the amount of about 10 to95% by weight. In one preferred embodiment, about 50% filler by weightmay be added to the polyimide core.

Adding the filler to the core 100 advantageously increases the thermalconductivity of the interposer. For example, adding filler to the corepreferably increases thermal conductivity by about 50%, more preferablyabout 500%, as compared to that of the polyimide core above. Forexample, whereas a polyimide film alone may have a thermal conductivityof about 0.2 W/mK, with preferred amounts of filler, the thermalconductivity increases to about 0.3 to 1 W/mK or more. More preferably,the thermal conductivity of the polyimide core with the filler is about0.5 W/mK or more. This therefore enables more efficient cooling of theinterposer during operation, and allows the device to be more accuratelytemperature-controlled.

Moreover, the filler advantageously decreases the coefficient of thermalexpansion (CTE) of the interposer to more closely approximate the CTE ofthe die. In one example, a polyimide core without a filler has acoefficient of thermal expansion of about 20 ppm/° C. By adding about30% of a SiO₂ filler, the CTE of the core is reduced to about 15 to 17ppm/° C. or less to more closely resemble a die having a CTE of about 3ppm/° C. In other embodiments, a composite polyimide core having a CTEof about 3 to 10 ppm/° C. may be formed. In general, filler may be addedto the polyimide core to reduce the CTE of the core by at least about10%, more preferably about 25%.

The reduced CTE of the interposer advantageously more closely matchesthe CTE of the die. This in turn reduces the stresses placed on the tape30, and more particularly on the leads 50, during thermal cycling. Theoverall package is thus more resistant to breakage of the wire and thecorresponding loss in electrical connection. Moreover, the reduced CTEof the interposer prevents delamination of the die attach layer 80 fromeither the die or the interposer. Additionally, in conventionalfabrication processes using a metal frame to carry the interposer, themetal frame typically is exposed to the entire assembly process,including the high temperature post die attach cure, wire bond,encapsulation cure and solder ball placement. Thus, reducing thecoefficient of thermal expansion of the interposer also more closelymatches the CTE of the interposer to the CTE of the metal carrier,which, for example, has a CTE of about 17 ppm/° C. when made of a coppermaterial. This therefore prevents the interposer from delaminating fromthe metal carrier.

The presence of the filler in the polyimide core 100 also preferablyincreases the rigidity of the interposer. For example, in oneembodiment, when about 30% of SiO₂ filler is used, the tape 100 has amodulus of about 10 GPa at 25° C. In other embodiments, the modulus mayrange from about 10 to 50 GPa.

The increased rigidity of the tape 100 advantageously makes the tapeeasier to handle during fabrication of the package. With conventionaltapes, having a modulus in the range of about 4.5 to 8 GPa for example,during assembly of the package the interposer is carried using a metalframe as described above. The composite interposer of the preferredembodiments, by contrast, has a higher modulus which may eliminate theneed to use a metal frame. For example, filler may be added so that themodulus of the interposer is about 5 and 500% higher than the modulus ofthe polyimide core alone. This thereby simplifies manufacture, and theincreased rigidity of the tape makes it possible to handle the tapedirectly by a machine without using a metal frame. Elimination of themetal frame helps the process accuracy and reduces handling and costs.

Moreover, the more rigid interposer of the preferred embodiments alsoprevents die delamination. This is because a more rigid interposer canbe made flatter and can therefore be adhered to the die attach materialmore effectively.

It will be appreciated that the interposer described herein may be usednot only in μBGA packages, but also in other integrated circuit packagesas well. Other types of integrated circuit package applications as wouldbe known by one of skill in the art include, but are not limited to, anypackage using a flexible substrate. Examples include Chip-on-flex, D²BGAand BOCBGA with flexible substrates.

The embodiments illustrated and described above are provided merely asexamples of certain preferred embodiments of the present invention.Various changes and modifications can be made from the embodimentspresented herein by those skilled in the art without departure from thespirit and scope of the invention, as defined by the appended claims.

What is claimed is:
 1. An interposer for an integrated circuit package,comprising: a polyimide core; and a thermally conductive, electricallynonconductive filler wherein the filler increases the thermalconductivity of the interposer by about 50% or more as compared to thatof the polyimide core alone.
 2. The interposer of claim 1, wherein thefiller is a nitride.
 3. The interposer of claim 2, wherein the filler isboron nitride.
 4. The interposer of claim 2, wherein the filler issilicon nitride.
 5. An interposer for an integrated circuit package,comprising: a polymide core; and a thermally conductive, electricallynonconductive filler, wherein the filler is aluminum nitride.
 6. Theinterposer of claim 1, wherein the filler is alumina.
 7. The interposerof claim 1, wherein the filler is silica.
 8. The interposer of claim 1,comprising between about 10% and 95% filler by weight.
 9. The interposerof claim 5, comprising about 30 to 50% filler by weight.
 10. Theinterposer of claim 1, wherein the thermal conductivity of the polyimidecore collectively with the filler is about 0.5 W/mK or more.
 11. Theinterposer of claim 1, wherein the thermal conductivity of the polyimidecore collectively with the filler is about 0.3 W/mK or more.
 12. Theinterposer of claim 1, wherein the filler reduces the coefficient ofthermal expansion (GTE) of the interposer by about 3% or more ascompared to the CTE of the polyimide core alone.
 13. The interposer ofclaim 1, wherein the filler reduces the coefficient of thermal expansion(CTE) of the interposer by about 10% or more as compared to the GTE ofthe polyimide core alone.
 14. The interposer of claim 1, wherein thecoefficient of thermal expansion of the polyimide core collectively withthe filler is about 17 ppm/° C. or less.
 15. The interposer of claim 1,wherein the filler increases the modulus of elasticity of the interposerby about 10% or more as compared to the modulus of the polyimide corealone.
 16. The interposer of claim 1, wherein the modulus of elasticityof the polyimide core collectively with the filler is about 10 GPa ormore.
 17. The interposer of claim 1, wherein the filler material has anaverage diameter of less than about 75 μm.
 18. The interposer of claim1, wherein the filler material is inorganic.
 19. An interposer for aball grid array package, comprising: a polyimide core; and a nitridefiller material wherein the filler material increases the thermalconductivity by about 50% or more, reduces the coefficient of thermalexpansion by about 10% or more, and increases the modulus of elasticityby about 10% or more as compared to that of the polyimide core alone.20. The interposer of claim 19, wherein the filler material is boronnitride.
 21. The interposer of claim 19, wherein the filler material issilicon nitride.
 22. The interposer of claim 19, wherein the filler isaluminum nitride.
 23. The interposer of claim 19, wherein the filler isalumina.
 24. The interposer of claim 19, wherein the filler is silica.25. An interposer for an integrated circuit package, comprising: apolyimide core; and a inorganic filler material wherein the fillermaterial increases the thermal conductivity by about 50% or more orcomposed to that of the polymide core without the inorganic fillermaterial.
 26. An interposer for an integrated circuit package,comprising: a polymide core; and a thermally conductive, electricallynonconductive filler wherein the thermal conductivity of the filler isabout 0.3 W/mK or more.
 27. The interposer of claim 26 wherein thethermal conductivity of the filler is about 0.5 W/mK or more.
 28. Aninterposer for an integrated circuit package, comprising: a polymidecore; and a thermally conductive, electrically nonconductive fillerwherein the filler reduces the coefficient of thermal expansion (CTE) ofthe interposer by about 3% or more as compared to the CTE of thepolymide core alone.
 29. The interposer of claim 28 wherein the fillerreduces the coefficient of thermal expansion (CTE) of the interposer byabout 10% or more as compared to the CTE of the polymide core alone. 30.An interposer for an integrated circuit package, comprising: a polymidecore; and a thermally conductive, electrically nonconductive fillerwherein the coefficient, of thermal expansion of the polyimide corecollectively with the filler is about 17 ppm/° C. or less.
 31. Aninterposer for an integrated circuit package, comprising: a polymidecore; and a thermally conductive, electrically nonconductive fillerwherein the filler increases the modulus of elasticity of the interposerby about 10% or more as compared to the modulus of the polyimide corealone.
 32. An interposer for an integrated circuit package, comprising:a polymide core; and a thermally conductive, electrically nonconductivefiller wherein the modulus of elasticity of the polyimide corecollectively with the filler is about 10 GPa or more.
 33. An interposerfor an integrated circuit package, comprising: a polymide core; and athermally conductive, electrically nonconductive filler wherein thefiller material has an average diameter of less than about 75 μm.